The invention is directed to a developer station in an electrophotographic means for the development of charge images applied to charge image carriers with the assistance of a developer mix, whereby the developer mix is taken from a supply chamber in a developer station by a transport drum and is transported to developer drums that rotate in opposite directions relative to one another and ink the charge image carrier.
In copier equipment technology and in non-mechanical fast data printers that operate based on the principle of electrophotography, charge images are generated on a charge image carrier, for example on a photoconductive drum, and are subsequently inked with a colored powder, toner in a developer station. The toner images are subsequently transferred onto normal paper given employment of a photoconductive drum and are fixed there.
As a rule, a two-component developer is employed for developing, this being composed of ferromagnetic carrier particles and of colored toner particles. The developer mix, for example, is conducted past the charge image on the charge image carrier with a magnetic brush arrangement, the toner particles adhering to the charge image as a result of electrostatic forces. The magnetic brush arrangement is thereby composed of a rotatable hollow cylinder in whose interior a plurality of rows of stationary permanent magnets are arranged. A plurality of magnetic brush arrangements can be provided in one developer station. For example, one magnetic brush arrangement can serve the purpose of transporting the developer mix past the charge image carrier, referred to below as developer drum. A further magnetic brush arrangement can be employed in order to transport the developer mix out of the inside of the developer station to the developer drum. Such a magnetic brush arrangement or any other arrangement that effects such a transport of developer mix is thereby referred to as transport drum below. Developer stations wherein developer mix for inking the charge images on the charge image carrier with the assistance of the magnetic brush principle are employed are disclosed by German Patent 31 19 010 and corresponding U.S. Pat. No. 4,461,232.
When developer drums are operated such that the developer mix is applied in the moving direction of the charge image carrier, one speaks of isorunning developer drums or co-running developer drums. When the developer mix is applied opposite the moving direction of the charge image carrier with the assistance of the developer drums, then such developer drums are referred to as counterrunning developer drums.
Developer stations that contain both counterrunning as well as isorunning developer drums are disclosed, for example, by US-A-3,912,388 and by US-A-3,881,446. The developer drums additionally have a metering means allocated to them that is composed of an adjustable blade and that serves the purpose of impressing a defined height or, respectively, thickness on the carpet of mix on the developer drums. The developer mix is thereby simultaneously offered to both drums from the supply space.
Developer stations having isorunning developer drums as disclosed, for example, by GB-A-1 524 543 have good printing results given line-like patterns such as, for example, characters. The inking is unsatisfactory given full surfaces such as occur, for example, in graphic illustrations or given black bars as needed, for example, when printing bar codes.
Numerous attempts have been made in order to be able to satisfactorily accomplish the full-surface inking.
Thus, a better full-surface inking without modification of the developer station can be achieved when the toner and/or the carrier material in the developer mix is changed. In all such instances, however, a considerable reduction of the useful life of the mix has hitherto also derived, this being a great disadvantage because of the higher costs for material consumption and maintenance connected therewith.
A further possibility of improving the full-surface inking is comprised in switching from the reversal development method wherein the discharged regions of the charge image carrier are inked to the direct development method standard in copier technology wherein the charged regions are inked. Given direct development, however, the majority part of the surface of the charge image carrier must usually be exposed. This largely excludes the employment of specific character generator principles such as, for example, LED lines or the employment of laser diodes because of the thermic problems connected therewith. However, it is precisely these two character generator principles that come into consideration when high point grid density given high printing process speed are required.
Further attempts for improving the full-surface inking have shown that no uniform surface inking can be achieved with a single developer drum given the established marginal conditions (composition of the toner mix, speed). On the contrary, wash-out effects at the trailing edge of the full surfaces given isorunning development or, respectively, at the leading edge of the full surface given counterrunning development are observed therein, these having an especially disturbing effect in critical image patterns, for example inverse printing.
Disturbances in the printer format, particularly light spots, form a further problem in such developer stations, these light spots deriving therefrom that the carrier particles contained in the developer mix are entrained by the photoconductive layer of the photoconductive drum up to the transfer station and deteriorate the transfer printing event. The standard magnetic carrier stripper drum is not in a position by itself to reliably avoid this disturbance.